A conventional manhole bored in a street opens through a top circular collar surrounded by concrete, which collar receives a circular cover closing the manhole and having a top surface that should be at the same level as the surface of the street. When a street, after years of wear, has to be repaved, e.g. with asphalt, the cover must be raise d to remain at the level of the street surface. Conventionally, this operation is carried out by digging, breaking the concrete around the manhole collar, and pouring concrete on the new foundation around the upwardly-offset collar. Of course, such an operation is tedious and time-consuming, and requires at least four workers as well as a heavy, pneumatic equipment. This conventional method of raising the cover level of a street manhole is therefore expensive.
In applicant's co-pending Canadian Patent application No. 5873,231 filed July 28, 1988, there is disclosed a ring1ike device for raising a circular, peripherally-notched cover of a street manhoie opening through an upper, circular collar peripherally surrounding the cover and supporting the same, which collar having a geometrical axis and comprising at least one inner projection fitting in one peripheral notch of the cover to prevent rotation of the latter about the above geometrical axis; this ringlike device comprises: a lower circular ring (a) with a top surface, (b) having a first diameter so dimensioned that the lower ring fits in the manhole collar, and (c) being bent to define at least one peripheral, outer notch so designed that said at least one inner projection of the collar fits therein; an upper, circular ring (a) with a top surface, (b) having a second diameter greater than the first one and so dimensioned that the cover fits within the upper ring, and (c) being provided with at least one inner projection fitting in the notch of the cover; and means for assembling together the circular rings, with the lower and upper rings coaxial to each other and with the top surface of the upper ring higher than the top surface of the lower ring; whereby, (a) the lower ring can be disposed in the collar after the cover has been removed therefrom, with said at least one projection of the collar fitted in said at least one notch of the lower ring to prevent rotation of the ringlike device about the geometrical axis of the collar and with the collar peripherally surrounding the lower ring and supporting the same, (b) the cover can be disposed on the top surface of the lower ring with said at least one projection of the upper ring fitted in said notch of the cover to prevent rotation of the cover about the geometrical axis of the collar and with the upper ring peripherally surrounding the said manhole cover, and (c) the street can be covered with an additional layer of pavement of a thickness corresponding to the height of the top surface of the upper ring above an already existing pavement, which additional layer of pavement surrounding and compressing the upper ring to hold firmly in position the ringlike, cover raising device.
However, in this co-pending Canadian Patent application, the tongues in such a device, since they are parallel to the through-axis of the circular rings, or the weld joints thereof about the circular ring, will break when the spacing adjustment between the circular rings is made under a substantial load being applied in the said direction of through axes of the circular rings. This thus renders inoperative a good number of ring-like members.